1. Field of the Invention
The invention relates to a process for the preparation of an ultra pure hydrogen peroxide solution, and to a plant for the implementation of the process.
2. Description of the Related Art
The use of hydrogen peroxide for advanced technology applications or applications in the food industry, in hygiene or health requires increasingly pure products which must meet a growing number of increasingly tight specifications. In particular, the demands of users are turning towards hydrogen peroxide solutions in which the content of each metal impurity is less than one part per billion (ppb) and preferably less than 100 parts per trillion (ppt). In the following account, such solutions will be known as ultrapure hydrogen peroxide solutions.
It is well known, according to the prior art, that it is possible to remove certain impurities by passing the solution through a bed of ion-exchange adsorbents. Mention may be made, for example, of functionalized polymers of polystyrene/divinylbenzene type, silicas or aluminosilicates, in particular the varieties containing controlled micropores, such as zeolites, or active charcoals; these solids carry functional groups capable of complexing either cations or anions. Mention may be made, as examples of functional groups capable of complexing cations, of the carboxylic, sulphonic, phosphonic, hydroxide, amine oxide or phosphine oxide groups or alternatively of cyclic or open polyoxaalkyls, such as, for example, ethylene oxide polymers. Mention may be made, as examples of functional groups capable of complexing anions, of the quaternary ammonium or quaternary phosphonium groups. These adsorbents can also be obtained by polymerization of a monomer carrying a functional group, for example poly(methacrylic acid)s, poly(vinylphosphonic acid)s, polyvinylpyridines, polyvinylpyrrolidones, poly(vinyl alcohol)s, saponified polylactones and copolymers containing these units. The adsorbents which are the most often described are polystyrene gels or crosslinked polystyrenes possessing sulphonic --SO.sub.3 H or trimethylammonium (CH.sub.3).sub.3 N.sup.+ functional groups.
Many combinations have been provided, such as, for example, anionic resin followed by cationic resin or cationic resin followed by anionic resin or alternatively anionic resin followed cationic resin followed by cationic+anionic "mixed bed". Additions to the inter-stage phases are also described, such as, for example, the addition of acid in order to modify the pH or the addition of chelating agents, such as aminomethylenecarboxylic or aminomethylenephosphonic derivatives.
It is well known to the person skilled in the art that the use of anion-exchange adsorbents presents great difficulties when employed for the purification of hydrogen peroxide. In particular, the hydroxide form, under which these products are generally available industrially, cannot be used directly because of its excessively high basicity, resulting in significant decomposition of hydrogen peroxide. Many publications describe the use of adsorbents exchanged by carbonate or bicarbonate ions, which are less basic, in order to limit the decomposition of hydrogen peroxide, without, however, eliminating it completely.
It is essential to be able to control this phenomenon of decomposition of hydrogen peroxide on adsorbent beds because, as this decomposition with release of gaseous oxygen is exothermic, the rate is accelerated according to the law of Arhenius. The formation of a gas pocket can further aggravate the phenomenon since, by separating the liquid from the decomposition point, the heat released can no longer be removed by evaporation of the water and the cooling effect of the liquid is lost. Such a process is characteristic of a divergent reaction which can result in an extremely violent autoaccelerated decomposition reaction, which is all the more dangerous since it is highly exothermic and produces gaseous oxygen, thus with a considerable expansion force which can cause explosions.
It has been shown that the phenomenon of initiation of the violent decomposition reaction by simple heating of a bed of anion-exchange resin in the trimethylammonium bicarbonate form (Dowex A 550 UPE) in the presence of a 30% aqueous hydrogen peroxide solution at moderate temperature, for example 30 to 350.degree. C., for a few tens of minutes is much faster with a resin which has been used for the purification of the peroxide than with a freshly exchanged resin. If the "TMR" (time to maximum rate), which indicates, at a given temperature, the induction period before initiation of the explosive decomposition, is taken as evaluation parameter, the following results were obtained:
fresh resin:
T=56.degree. C.: TMR=15 min; T=51.degree. C.: TMR=30 min; T=44.degree. C.: TMR=60 min
used resin:
T=41.degree. C.: TMR=15 min; T=35.degree. C.: TMR=30 min; T=32.degree. C.: TMR=60 min
It is thus obvious that a used resin is much more sensitive to hydrogen peroxide autodecomposition phenomena, probably because of the catalytic effect of the metal species exchanged during purification.